While wood has many desirable qualities that make is useful for structural members, use of sawn lumber for structural members also creates several difficulties because of some inherent problems. First of all, wood timbers are inherently nonuniform in their structural characteristics. The presence of knots and the location thereof from one structural member to another can cause great variation in the structural strength of a member. The location of the wood of a structural member within a tree can cause a variation in its characteristics from a member that is taken from a different portion of the tree. Moreover, high grade structural quality wood timbers are becoming increasingly more expensive as the supply of old growth, virgin trees nears exhaustion. The second growth trees from which more and more lumber is originating tend to have more knots and other defects which makes it less suitable for structural purposes.
Because of the wide disparity in the strength of wooden structural members, several difficulties in the use of such members are created. First, the structural members must be carefully graded, and any members that have apparent weakening defects must be rejected or downgraded which, of course, decreases their commercial value substantially. Second, because of the increasing scarcity of high grade wood structural members, they are becoming increasingly more expensive. Moreover, because of the wide variation in structural strength existent even within a carefully graded lot of wooden structural members, in order to ensure an adequate safety margin, larger members or an increased number of members have to be specified than would be the case if the structural strength fell within a narrower range.
Previous attempts to increase the strength of wooden structural support members have been made. For example, U.S. Pat. No. 3,717,886 discloses a bed frame with reinforced slats consisting of a flat, rolled steel reinforcing member attached to the bottom face of a wooden slat member. In U.S. Pat. No. 3,294,608 a wood beam is prestressed and a steel plate bonded to the surface under tension. However, although suitable for use in small scale applications, such systems could not function economically under large-scale construction conditions. Besides the high cost of manufacture and the additional weight, such composites would present fastening problems and are not adapted to be cut to shorter lengths with the usual wood-working equipment. Likewise, prestressed elements have been used to reinforce structural members. For example, U.S. Pat. No. 3,533,203 discloses the use of stretched synthetic ropes to apply a compressive force to such diverse items as concrete beams, aluminum pipe and ladder rails, the stretched element being attached by clamps or similar means to the member. U.S. Pat. No. 3,890,097 discloses the manufacture of fiber board wherein fiberglass strands are embedded in the matrix as the board is laid up and held under tension until the resin has set and in U.S. Pat. No. 4,312,162 tension is applied to steel or fiberglass strands laid up along the side of a fiberglass light pole until a resin matrix sets to bind the strands of the pole.
In U.S. Pat. No. 3,251,162 a series of rods or cables pass through a laminated beam and are connected to tensioning plates and bolts at either end. Similarly, in U.S. Pat. No. 3,893,273, a vertical rod tensioned at either end is set in the edge of a door. U.S. Pat. No. 4,275,537 discloses a whole series of truss assemblies composed in each case of multiple parts, in which the basic principle is the use of pre-stressed or pre-loaded elements, such as tensioned cables or steel straps to accomplish reinforcement.
These prior procedures and products each have inherent disadvantages. The disadvantage of steel and like reinforcing material has already been discussed. The manufacture of products where one or more elements must be held under tension is inherently expensive. In constructions of multiple parts, a total product is produced, such as a ladder, a door or a truss which must be used as a whole. Thus, none of the patents cited permit easy cutting to size at the job site to suit the needs of the job.